lib/pnacl: fix import of extern classes types source files

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: pointers are equal if their address are the same

This was the previous behavior but it was at a lower level.

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
model: intro a service to get the `Pointer` type

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg*: extern classes a polymorph in Nit, and unboxed only for extern methods

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>